A power network comprises a plurality of interconnected producers and consumers of electric power. A producer comprises an electric power generating equipment such as a generator and a consumer comprises a power consuming equipment such as a motor or a furnace. The network also comprises a transmission net, which is the media in which the electric power is transported from a producer to a consumer. A fault condition, which may be caused by a sudden current current rush, in such a network will result in a voltage drop in the transmission net. Apparatus connected to the net all have safety equipment which senses the voltage in the grid point and on sensing a voltage drop rapidly disconnects the apparatus from the transmission net.
For a rotating electric machine a sudden rush of current would instantaneously increase the heat generation in the electric circuit. This increase in heat would harm the machine in a matter of less than part of a second. Therefore a rotating electric machine is protected for a sudden current rush by a control means such as a switchgear, which immediately disconnects the machine from the connection to a grid point. Thus the electric circuit will be open such that no current can flow. A rotating electric machine is often connected or integrated with a mechanical machine. Thus by supplying a mechanical force such as hydro power, wind power or the power from a combustion engine to the mechanical machine the electric machine is rotated and thus producing power. In this embodiment the rotating electric machine is a generator.
A system of a rotating electric machine integrated with a mechanical machine can be seen as a mechanical circuit, a magnetic circuit and an electric circuit interacting with each other. Thus by disconnecting one of those circuits the other two have to be disconnected too. While the electric power can be disconnected in matter of seconds the mechanical power cannot be disconnected that fast. Often there is a large moment of inertia incorporated in the mechanical machine, which must be affected to stop the machine. Thus there must be equipment present, like a brake, to make possible to stop the rotating electric machine. Also the magnetic circuit involves a moment of inertia and the saturation of the iron core of the stator or rotor or if there is present a permanent magnet. In such cases the still revolving electric machine will produce electric energy which could produce partial discharges harming the insulation of the electric winding of the rotating electric machine. Also there are the mechanical power supply, like wind flow or water flow which cannot be diverted or stopped at all or at least not that fast.
Whether the rotating electric machine is a generator, thus producing electric power or a motor thus producing mechanical power the disconnection of the electric circuit affect the two other circuits in a way that a plurality of safety arrangements have to be present in both cases.
A transformer can be seen as a first electric circuit, a magnetic circuit and a second electric circuit interconnected. A sudden rush of current would also in this case lead to an instant increase in heat in the electric circuits and thus be harmful. Both the electric circuit can be instantaneous disconnected from each of their connections. Depending on when in matter of milliseconds this disconnection takes place there might be a magnetically stored energy hidden in the transformer core. This implies that also transformers must have a plurality of safety arrangements in order to protect the windings of the transformer.
All apparatus for controlling the network, such as power electronics, reactors and capacitor banks, have inherent stored energy which must be taken care of in a situation of over current and disconnection. Also in these situations there are safety arrangement for disconnecting the apparatus from the net and protection against the stored energy. High voltage capacitor banks normally have their capacitor units individually fused. Capacitor bank feeders are normally protected by fused contactors or switches. In the case of circuit breakers, phase fault and earth fault protection are provided.
The starting and stopping of constant speed asynchronous induction motors, as required by the manufacturing or plant process, is the most common control function in any industry. For this reason, a fused switch combined with a contactor and some minor protective and auxiliary relaying is, in some parts of the world, given the name “motor controller”. Similarly, an assembly of such units is likewise given the name “Motor Control Centre” or MCC. Starting and stopping may only require manual operation, however, MCC are normally under the management of a computer, which may execute the start and stop actions without interference from operators. Sometimes the process operator may wish to overrule the computer and start or stop motors manually, providing it is safe for the process to do so.
Motor control may be more sophisticated and include the variation of speed, traditionally done with DC motors. More and more often the control is effected by adjusting the frequency to either cage-type induction motors. For larger units the control is effected by synchronous motors all the way up to 40 MW for large compressor drives. For such large ASD (adjustable speed drives) it is essential to recognize that the speed controller (or frequency converter) is in integral part of the motor package, where all parts are finely tuned to each other. The interface with process control is basically only to provide a protocol for an input signal to the frequency converter in order to increase or reduce speed.
Synchronous motors resemble generators and therefore some generators protection schemes may also be used for synchronous motors.
Whether at low or medium voltage, a motor circuit supplied either via circuit breaker or a fused contactor. This is the “last” over current decide in a series of over current devises. There are two types of protection required for a motor circuit. First of all, the motor and feeder cables are protected against a short circuit by the circuit breaker or fuses. Secondly, protection is required to prevent an increase in load causing excessive current and heating in the motor, cable, and associated switchgear controlgear.
Transformer protective relaying is first of all provided to limit the consequences of faults and failures such as a short circuit inside the transformer and in the connecting leads. Such faults are very rare, but if such failure should occur, it may develop very fast, such that the protection cannot save the transformer from permanent damages. A fast disconnection will, however, limit the results preventing a devastating fire or explosion. Large transformers have further protective arrangements such as redundant or duplicated short circuit protection, under impedance and differential protection. Delayed overvoltage protection is provided for transformers with a risk of elevated voltage, which can cause core magnetic saturation and overheating damages if permitted to last.
High voltage capacitor banks normally have their capacitor units individually fused. Capacitor bank feeders are normally protected by fused contactors or switches. In the case of circuit breakers, phase fault and earth fault protection are provided.
From U.S. Pat. No. 6,411,067 (Björkman) is previously known a voltage source converters operating either as back-to-back stations or as parallel static var compensators. The object of the converter arrangement is to provide a device for controlling the flow of electric power in a transmission line carrying alternate current.
It is further known from the document that in an electric transmission system it is of great importance and value to be able to rapidly and precisely control the flow of electric power so as to adapt the power flow to varying load conditions and to achieve a stable and predictable power flow despite disturbances of different kinds. Different types of devices have been proposed and put into operation for achieving this control of power flow. A device commonly used for this purpose is the so-called Unified Power Flow Controller (UPFC). The UPFC consists of two AC/DC voltage source converters designated as exciter and booster, respectively. The DC sides of both converters are connected to a common capacitor providing a DC voltage support for the converter operation and functioning as an energy storage means.
The AC side of the booster inserts a synchronous AC voltage of controllable magnitude and phase angle in series with the transmission line via a series transformer. The AC side of the exciter is connected in parallel to the transmission line via a transformer where a current of controllable magnitude and power factor angle is injected into or absorbed from the transmission line. By means of a UPFC, the active and the reactive power flow through the transmission line can be controlled independently of each other. The main task of the exciter is to control the DC link voltage and to keep it on the reference value by exchanging the specific amount of active power with the transmission line. The secondary task of the exciter is to compensate reactive power as a var compensator so as to keep the line voltage on a constant level.
It is thereby known in order to provide a device highly effective for controlling the flow of electric power in a transmission line carrying alternating current, which device can be manufactured at relatively low costs. Such a device comprises a first VSC (VSC=Voltage Source Converter) connected to the transmission line at a first point and a second VSC connected to the transmission line at a second point, said first and second VSC having their DC sides connected to a common capacitor unit, wherein the device further comprises a by-pass switch connected to the transmission line between said first point and said second point in parallel with the first and second VSC so that the first and second VSC will operate as a back-to-back station when the by-pass switch is open and as two parallel static var compensators when the by-pass switch is closed.
When the by-pass switch of the device is open and the VSC operate as a back-to-back station, a powerful control of the flow of electric power in the transmission line can be achieved. During this first mode of operation, the phase, the frequency as well as the magnitude of the alternating voltage in the transmission line can be controlled by means of the device, and the active and reactive power can be controlled independently of each other. When the by-pass switch of the device is closed the VSC operate as two parallel static var compensators. During this second mode of operation, the device indirectly controls the transmission line voltage, and thereby the transmitted electric power, by generating reactive power for, or absorbing reactive power from, the transmission system. The device is preferably operated in said first mode during time periods when the flow of electric power in the transmission line has to be controlled to a large extent, whereas the device is preferably switched over to said second mode during time periods when only minor regulations or no regulations at all of the power flow are required. When the device is operated in said second mode, the power losses are lower than during operation in said first mode.
From U.S. Pat. No. 6,512,966 (Löf et al) is previously known a method for enhancing a commercial value of a unit of electric power produced by a renewable power production facility. The object of the method is to enhance commercial value of electrical power produced from a renewable energy power production facility
The document further states the wind power is a “natural” power production source that instinctively should be regarded as an optimum source of energy for producing electric power. Wind power does not require the burning of fossil fuels, does not result in nuclear waste by-products, does not require the channeling of water sources, and does not otherwise disturb the environment. On the other hand, wind power is a variable (stochastic) power generation source, thus not offering power production facilities the type of control that the power production and grid facility would like to have in producing commercially reliable power. To address this variability issue, even the early pioneers of wind power attempted to identify ways to “store” wind generated electric power in times of excess, so as to later compensate for times when there are lulls in the wind.
In the early days, wind energy plants were generally isolated from one another and provided small scale generation facilities. Through a variety of experiments wind energy plants have generally evolved and now a common theme is to group a number of wind turbines together so as to form farms that can generate up to tens of megawatts via the aggregation of smaller plants that produce slightly above only one megawatt each.
The method thus comprises identifying a predetermined amount of power predicted to be produced from the renewable power production facility at a predetermined future time and converting the predetermined power from the renewable power production facility to a unit of premier power for application to a power grid at a standard frequency.
From U.S. Pat. No. 6,577,108 (Hubert et al) is known a voltage regulation of a utility power network including generation systems, transmission systems and distribution systems serving loads. Especially the regulation is related to a system for controlling the transfer of energy to and from a utility power network. The object of the regulation system is to compensate for power shortfalls or voltage instability problems on the network.
The system therefore includes a controller that controls a reactive power compensation device to deliver, for a first period of time reactive power to the utility network. In a second period of time, following the first period of time the controller controls the reactive power compensation device to provide reactive power to the network at a predetermined level. The power compensation device has a steady-state power delivery characteristics.
Having detected and reacted to a change of a predetermined magnitude in the nominal voltage on the utility power network by increasing injected power to a level that is as much as N (N>1) times higher than the maximum steady-state power delivery characteristic of the compensation device, power injection of the compensating device can be purposefully and gradually reduced to the maximum steady-state value so as not to include a transient response by the network that could result in voltage instability and/or other undesirable events.
The voltage regulation provides an approach for operating a reactive power compensation device in an overload mode for a maximum period of time without incurring an abrupt, step-like change in inverter current at the time the overload capability of the compensating device has been expended, thereby forcing the compensating device's current to be at or below a specified level. Thus, as noted, the invention reduces the possibility of undesirable transients (e.g., ringing oscillations) in the utility power network. Furthermore, a substantially optimum ramp down profile can be determined on the basis of the characteristic impedance of the network.
During the first period of time, the compensation device provides real power and reactive power to the utility power network. After the second period of time, the reactive power from the compensation device is non-discontinuously decreased to the steady-state power delivery characteristic. The factor N is generally determined on the basis of a transient thermal capacity characteristic (e.g., a 1% rating) of the compensation device. The second period of time is determined on the basis of the ability of the compensation device to absorb thermal energy. The ramp down profile may be determined on the basis of the characteristic impedance of the network. The characteristic impedance of the network may be determined using known characteristics of the network. Alternatively, the reactive power compensation device can apply a stimulus to the network and a response measured.
As show in the prior it is previously known systems and devices for protecting devices connected to a net, converters for controlling the electric flow, economic aspect to control a network and voltage regulation devices for compensation of power shortfalls or voltage instability. For the function of a power network itself there is a desire to keep the power production alive and the power transmission as well as the power consumption maintained. In cases of a fault condition in the net, however, all apparatus have a tendency to disconnect themselves from the net thereby leaving the net out of transmission and out of control. There is thus a long time need to provide a network that will still be alive and controllable in a fault condition.
There are regulations on national levels that demands for a wind farm to stay connected with at least some reactive and active power input to the grid during faults and to resume power production when faults are cleared. Further there are international specifications of wind farms connected to a transmission network. These specifications specify that offshore wind farms—like other major production plants—should not lose stability or trip at short-circuits in the network disconnected by the primary network protection. Said in a popular way, the turbines must be able to survive a short dead time (˜100 milliseconds) and resume production when the fault has been disconnected and the voltage starts to return.”
Wind turbines for producing electric power from the wind are even more delicate in this matter than other electric power producers. The wind cannot be cut off but is there for making electric power at every instant when a wind is present. If thus a wind mill is disconnected from the net the possible energy production will be lost. The wind will just pass the wind turbine whether it is propelling or not. There is thus great economical interests to keep the wind power production even in harsh conditions. Thus there is a need to have the wind turbines active even in a fault condition on the net. As the wind is a non storable energy every second counts.